The present invention concerns a high-temperature lubricant for the hot shaping of high-grade and carbon steels, which has a content of graphite, organic blowing agent and inorganic separation agent. The invention further concerns the use of the high-temperature lubricant according to the invention.
In the production of seamless pipes a solid steel block is heated to a given shaping temperature and pierced to form a thick-walled hollow block. That hollow block is fitted over a tool, the mandrel bar, and rolled out or stretched without the additional supply of heat over the mandrel bar by means of rollers bearing against the outside thereof. In that situation, in part considerable pressures and frictional forces occur on the surfaces of the metal to be worked and the working tool, and they crucially influence the durability of the working tools. In addition the frictional conditions ultimately determine the surface quality of the articles produced.
In the above-mentioned production of seamless pipes the most widely varying steel qualities are shaped, for example alloyed and highly alloyed steels and carbon steels. Different demands are made on separation agents and mandrel bar lubricants used, for those different steel qualities and for different wall thicknesses to be rolled out and stretching effects.
In consideration of the highly different material properties of the various steel material groups during the shaping procedure in the heated condition, the shaping of carbon steels mainly requires a lubricating action which is as good as possible on the part of the lubricant and, when dealing with alloyed and highly alloyed steels, in addition a separation effect which is as good as possible in respect of the lubricant is needed. In addition, high demands are made on a high-temperature lubricant in relation to temperature stability and constant rolling results upon a change in the wall thickness and/or the quality of steel.
Rolling material is to be prevented from adhering to the tool surfaces as that gives rise to serious losses in quality on the inside surfaces of the finished products. Other important considerations in relation to the shaping operation which takes place after the tools are coated with the lubricant are adhesion of the lubricant to the tool, which is as good as possible, a rapid drying effect and uniform formation of the layer consisting of applied lubricant.
When dealing with carbon-bearing lubricants such as for example graphite-bearing lubricants, so-called cementation of the rolling material can occur in the region of the grain boundaries of the rolling material upon contact with the lubricant at shaping temperatures of the order of magnitude of 1100 to 1300° C., in which case carbon diffuses into the surface of the metal and the situation can involve partial embrittlement of the metal and the formation of holes with penetration depths of up to about 300 μm. Upon further processing of the workpiece, the result of embrittlement of the metal is that the embrittled material tears apart and the workpiece becomes useless. Holes which are formed in that way are rolled out to form longitudinal scoring marks or lines in the subsequent elongation procedure. Those scoring lines represent a considerable, unacceptable reduction in quality of the finished rolled material, which has to be avoided.
Depending on the respective quality of steel and rolling process involved, different, especially adapted lubricant compositions have already been developed. The consequence of this is that, upon changes in production, for example when changing from alloyed and highly alloyed steel qualities to carbon steels, the lubricant also has to be replaced, besides the changes in process settings and rolling parameters, which have to be carried out by the rolling mechanism operator. That involves cost-intensive disadvantages, such as for example longer interruptions in production, an increased amount of work due to the conversion operations involved, the necessity to store widely varying lubricants which are matched to the material and the rolling process, the provision of additional mixing and storage containers, and the provision of a further separate apparatus for applying the alternative lubricant which is suited to the specific requirements involved.
Lubricants for the area of use of high-grade steel shaping are described for example in EP-A-0 357 508. However, they are optimised for that area of use and are therefore not the optimum for shaping carbon steels, in relation to tool service lives and current consumption levels in respect of the rollers.
Lubricants for the field of carbon steel shaping are described for example in EP-A-0 164 637, EP-A-0 554 822 and EP 0 909 309. More highly alloyed materials can be shaped with those lubricants, under comparable conditions, only when a deoxidation agent in powder form additionally assists with the lubrication effect.
EP 0 745 661 discloses a graphite-bearing lubricant which has a proportion of one or more clay minerals from the class of smectites. In addition those lubricants have either a content of silica sol or potassium aluminium silicate. In accordance with EP 0 745 661 such lubricants can very substantially overcome the disadvantage of cementation of graphite-bearing lubricants at relatively high working temperatures.
Admittedly, such lubricants with contents of graphite and sheet silicate can exhibit a lesser degree of cementation in metal working operations, but it will be noted that they are frequently in need of improvement in regard to the frictional conditions between the metal surfaces in order to prolong the durability of the working tools, for example the mandrel bars.
Therefore the object of the present invention is to provide a high-temperature lubricant which can be used for a wide range of steel qualities for different wall thicknesses to be rolled and stretching effects and which is moreover stable in respect of temperature, provides constant rolling results upon a change in the wall thickness and/or the quality of steel and does not lead to unwanted cementation of the rolled material. A high-temperature lubricant of that kind has hitherto not been described.